Technical Field
The present invention relates to optical polarization splitters and, more particularly, to polarization splitters operating on telecommunication wavelengths.
Description of the Related Art
Photonic structures can be fabricated on wafer chips in order to create wafers that operate both in an electronic domain and an optical domain. When an optical fiber is used to input light into a waveguide on a wafer chip, care must be taken to properly manage the polarization of light. The orientation of the polarization state in an optical fiber changes randomly with time. The performance of photonic devices on wafer chips is very sensitive to the orientation of the polarization state. Hence, the input polarization state must be processed on the wafer chip for it to be re-oriented into the polarization state for which the photonic devices work the best. To achieve such polarization re-orientation, a polarization splitter and rotator (PSR) is used.
Polarization management is a key technology in integrated photonic circuits. Two orthogonal polarizations of a signal are separated and treated separately on-chip. There are several different designs that are used to split the polarizations using on-chip photonic structures and waveguides, each with distinct disadvantages.
In a first polarization splitter, known as a directional coupler, a vertical polarization (denoted as “TM” for the “transverse magnetic” mode in a waveguide) couples more strongly with a splitter waveguide than a horizontal polarization (denoted as “TE” for the “transverse electric” mode). By bringing the input waveguide into proximity with the splitter waveguide, the TM polarization is removed from the input waveguide and propagates within the splitter waveguide, while the TE mode continues in the input waveguide. However, these structures have a narrow optical bandwidth, high sensitivity to fabrication imperfections, and obtaining low crosstalk necessitates cascading many directional couplers.
A second polarization splitter, known as a grating coupler, introduces a signal in a direction perpendicular to the split outputs through a grating. The TE and TM polarizations are scattered in different directions by the grating. This enables vertical coupling to the optical fiber, but again is limited in optical bandwidth and is sensitive to the grating dimensions.
A third polarization splitter, shown in a top-down diagram in FIG. 1, is known as a mode-evolution polarization splitter and uses two waveguides of differing thickness. A first waveguide 102 has a vertical size that is relatively larger than that of a second waveguide 104, while having a horizontal size that is relatively narrower. As the horizontal width of the first waveguide 102 is increased, TM polarized light (shown as a dashed line) becomes confined there while TE polarized light (shown as a solid line) remains confined in the second waveguide. This structure is relatively broadband and tolerant to variations in waveguide dimensions. However, when integrated into a microelectronic fabrication process and operating at the short-end of telecommunication wavelengths (e.g., in the 1.2 to 1.3 micrometer range), the point at which the TM input crosses from the second waveguide 104 to the first waveguide 102 is very sensitive to fabrication variation of waveguide 102 and appears close to a minimum feature size that can be reliably fabricated. For example, the splitter may not function correctly if the first waveguide 102 is a mere 20 nm too large.